# Matter & Interactions I, Week 8

**Posted:**October 14, 2016 |

**Author:**Joe Heafner |

**Filed under:**Course Log, Goals, Motivations |

**Tags:**metacognition, momentum principle, teaching |Leave a comment

This post was written one week late.

This week was devoted to chatper 3 of Matter & Interactions. The fundamental physics still centers almost exclusively on the momentum principle. Student are seeing more ways to apply it.

I’m asking quite a lot of my students. I expect them to read an entire chapter, doing the checkpoints in situ as they read, and work a parallel set of WebAssign problems in a single week (I define a week as beginning on Monday and ending on the following Sunday). That’s a lot. Sometimes I’m ridden with guilt knowing that most of my students won’t follow this schedule because they have other classes, jobs, families, or some combination of these and other obstacles (recall one student this semester is not fluent in English and is struggling because of that). However, I am expected to make my courses at least as rigorous as those in a four year institution and I like to think I set goals that are somewhat higher, at least for my students. By that I mean that sometimes my goals are different, and not necessarily harder to achieve, from the goals in a corresponding class at a four year school. Working textbook problems isn’t the be-all-end-all to learning introductory physics. Unfortunately, though, it’s how too many introductory physics are judged and I think that is quite unfair. I want my students to be able to talk about the underlying physics. I want them to think in terms of systems, system boundaries, and interactions crossing system boundaries. I want them to eventually recognize why we speak of a system’s momentum, energy, and angular momentum in particular. In addition the reading, I expect them to bring questions caused by the reading to class with them for discussion. I begin every class meeting with something like, “So what questions have been raised as you read through this chapter?” and I’m almost always met with total silence. I jokngly tell them that I treat silence as either total comprehension or total cluelessness knowing that it’s usually the latter. It gets a laugh, but I’m serious. I then morph the lead into something like, “What particular problems gave you trouble so far?” and this usually get a more concrete response. The trouble is almost always one of two things: **not knowing where to begin** or **not getting “the answer” that WebAssign expects**.

I’ll address the second one first. It is almost always my experience that the error can be traced to a calculator. I want students to completely abandon traditional calculators but that’s proving to be quite difficult to do. We’re all so used to carrying them around with us, even on our phones, and such habits are hard to break. It’s annoying, but this problem isn’t my main concern.

The bigger problem is when students perceive that they don’t know where to begin with a new physics problem (I don’t like the work “problem” and I really want to replace it with something more like “situation” or “vignette” or…I don’t know). In one instance this week, a student cited a problem he found particularly difficult. The problem asked to calculuate the vector gravitational force on a planet due to a nearby asteroid. Numerical masses and the objects’ mutual separation were given. The student was completely stymied. I mean he REALLY didn’t know where to begin. I was quite disturbed because this particular problem didn’t even use the momentum principle. It represents what I call a “sideline skill” type of problem. It’s something of a routine nature that is necessary for doing other things, except in this case the problem didn’t go that far. It’s just calculating a gravitational force. After more attempts on my part to lead the student to a starting point, he was still utterly lost (hopefully I didn’t cause that). So, I began a slow in situ analysis, modeling what I wanted him to do. I wrote on the board how a vector can be factored into a magnitude and a direction, and that at least for gravitational forces, Newton gave us a simple expression for the magnitude; the direction must come from simple geometrical reasoning. To me, the hardest part of this problem was calculating the relative position of one object from the other to get the correct direction vector. I don’t consider substituting numbers and units into the expression for the magnitude to be physics…it’s numerical substitution. So I worked the problem on the whiteboard and the student was quite satisfied afterwards (of course I did all the work, but whatever). Still, I’m not so naive to think that this student is where he needs to be; he clearly isn’t, and that bothers me. It REALLY bothers me, to the point where I almost blame myself for his being lost. Sometimes I care too much, but I can’t help it. Other students in the class weren’t stymied by this problem the way he was. That’s at least some reassurance to me that I probably didn’t cause his confusion. But oooooooh the guilt.

I remind student daily to not wait till the weekend to begin the WebAssign sets, but they almost always ignore this advice. Again, this causes me guilt but I realize that ultimately, they have a choice whether or not to take my advice.

But oooooooooooooh the guilt.

Let me know if you have suggestions for dealing with the guilt.